cpu/$(CPU)/Makefile.features and cpu/$(CPU)/Makefile.dep are
automatically included
Part of moving CPU/CPU_MODEL definition to Makefile.features to have it
available before Makefile.include.
FLASHER and FFLAGS are evaluated by the main Makefile.include or by file
included by it. Their value does not need to be exported.
This will also prevent evaluating 'PORT' for FFLAGS when not needed.
Testing
-------
`git diff --word-diff` only reports `export` being removed.
`git show --stat` reports `84 insertions(+), 84 deletions(-)`
Which is the same amount as lines that where matching
`export[[:blank::]]\+VARIABLE`.
Sometimes boards/*/Makefile.include (e. g. in case of the msba2) gets included
twice somehow, leading the TERMFLAG to be set twice and faulty. This
fixes that.
We have sane defaults for `ELFFILE` and `HEXFILE` in the root
`Makefile.include`. The local definition for `ELFFILE` of mbed_lpc1768's
`Makefile.include` was wrong, which caused e.g. `make buildsize` to
fail.
